Traveling wave tube assembly



July 30, 1968 WALLACE ET AL 3,394,453

TRAVELING WAVE TUBE ASSEMBLY Filed Oct. 4, 1965 mm a M ow m mm mm 0% 0V v mum M m mm m m w Wm mm mm mm vw 9v 9v WM A m R M mm I wmmm m Qllllllllllllllllllll/ United States Patent O 3,394,453 TRAVELING WAVE TUBE ASSEMBLY Milner W. Wallace, Saddle River, N..l'., and Richard C.

Wertman, Allentown, Pa., assignors to International Telephone and Telegraph Corporation, Nutley, N.J.,

a corporation of Maryland Filed Oct. 4, 1965, Ser. No. 492,594 8 Claims. (Cl. 29-600) ABSTRACT OF THE DISCLOSURE The helix, dielectric rods and shell structures of a traveling wave tube are assembled by use of a thermal shrink fit method and apparatus. The shell and helix are mounted on two axially aligned supports with the rods mounted on the helix in a fixed relationship. The shell is heated to expand uniformly to accommodate the slightly larger rods and helix which are positioned in the shell. The shell is then cooled to contract and secure the rods and helix in position with no' noticeable distortion of the members.

the intermediate dielectric rods and other member to provide a tight fit securing the components in a fixed position. Examples of such methods are found in US. Patent Nos. 2,943,228 issued June 28, 1960, No. 2,943,382 issued July 5, 1960 and No. 3,276,107 issued Oct. 4, 1966, the latter being assigned to the same assignee as the instant application. These devices have had some disadvantages in that the members retain excessive distortion after processing and there may be imprecise positioning of the related elements. For smaller diameter systems the distortion is proportionately much larger. In addition, the associated apparatus and atmosphere may introduce or permit undesired contamination.

It is therefore the primary object of the present inventionto provide an improved reliable method for assembling the shell-helix structure of a traveling wave tube.

Another object is to provide apparatus for securely mounting the shell-helix-rod assembly with minimum distortion of the members.

A further object is to permit assembly of the elements in a relatively clean atmosphere to prevent contamination.

These objects and advantages are achieved by a novel method and apparatus wherein the shell and helix are mounted on two axially aligned supports and placed within an evacuated enclosure. The shell is inductively heated by an external coil to uniformly expand and accommodate the slightly larger diameter helix and dielectric rods which are drawn into the shell by movement of the associated support tool. Upon cooling of the shell, the di ameter contracts and securely grips and positions the helix and surrounding rods. Use of a vacuum enclosure provides a pure atmosphere with the assembly being free of grease and other contaminants and in a cleaner condition than the separate parts before processing. The special tool also permits insertion of the helix and rods with minimum distortion of the shell. The details of the invention will be more fully understood and other objects and advantages will become apparent in the fol- 3,394,453 Patented July 30, 1968 lowing description and accompanying drawings wherein;

FIG. 1 shows a fragmentary cross-section of a vacuum jar andheating coil enclosing a shell holder and mounting fixture, and

FIG. 2 shows a fragmentary cross-section of the insertion tool which supports the shell, helix and rods and assembles the elements in the desired relationship.

As shown in FIG. 1, a first portion of the mounting fixture includes an intermediate tubular support which fits into a central opening in a cover 12. A journal 14 encloses the opening in thecover, with a small central hole 16 extending therethrough. The tubular support, cover and journal are joined by a vacuum tight heliarc weld 18. One end of a shell holder 20 is similarly welded at 22 to an adapter 24 which in turn is welded to the other end 26 of tube 10. A bottom plug 28 is press fitted into the other end of the shell holder 20 at an intermediate point along the length of the holder and has a central hole accommodating the inner helix, outer shell and inter-mediate dielectric rods. Peripheral slots 30 permit free pumping of air during evacuation. A plurality of cylindrical spacers 32 and centering wafers 34 prevent bowing of the shell. The waters have similar slots to those of plug 28 and are alternated with the spacers to make up a desired tube length for holding the shell and enclosing the helix and rods, with an end plug 36 enclosing the other end.

A second portion of the assembly, as shown in FIG. 2, forms a helix and shell insertion tool which includes a first mandrel 38 having a diameter permitting passage through the outer shell 4-0. Three equally spaced peripheral holes in one end of the mandrel accommodate the dielectric rods and form longitudinal slots along the end so that the diameter of the rods 42 protrude slightly beyond the diameter of the mandrel 38. This permits the shell to grip the rods independently of the mandrel during the assembly process as will be shown hereinafter. A tubular pull rod 44 is fitted over the other end of the mandrel 38 and welded thereto through a welding slot 46. A cylindrical plug 48 is inserted into the other end of the pull rod to provide a solid handle area to withstand pressure from tools which may be used for pulling the rod. A second mandrel 50 for mounting and inserting the helix is fitted into a central hole at the first end of the shell mandrel 38 and welded thereto through slot 52. Another slot 54 along the same end accommodates the end wire of the helix 56 to be mounted thereon. An alignment bushing and clamp 58 fits over the other end of the helix mandrel and provides holes identical to those in the shell mandrel 38 for accommodating and aligning the dielectric rods 42 and mandrel 59. The longitudinal slots in the bushing again permit the rods to contact the shell during assembly and facilitate the removal of the mounting fixture. An additional hole 57 in the end of bushing 58 accommodates the other end Wire of the helix. A clamp 60 fits into a groove 62 at the far end of the bushing and a screw 64 holds the various elements in place.

Before forming the complete assembly, all of the parts are degreased and chemically cleaned. Cleaning need not include vacuum firing as this is accomplished during the processing. The ceramic rods have a pyrolitic graphite coating for RF. attenuation which is applied separately. The assembly is then preferably accomplished in a clean area having a controlled atmosphere. One end of the helix mandrel 50, without the alignment bushing, helix or rods, is inserted into an opening 16 in the threaded end of the journal 14 and pushed through the cylindrical support 10 and shell holder 20, until the outer end of the pull rod 44 extends only a fraction of an inch out of the journal. O-rings 66 and washer 68 are placed over the exposed end of the rod and a nut 70 is screwed down until the rings are tight around the rod. The other end of the helix mandrel 50 extends the full length of the helix and dielectric rod assembly below shell holder 20 Without touching the bottom of the jar. The shell structure 40 is then slipped over the other end of the helix mandrel 50 and pushed into the shell holder 20 until it rests on a shoulder 72 at bottom plug 28. The helix 56 is then mounted on the helix mandrel 50 with one end resting against the shell mandrel 38 and the end wire inserted into the slot 54 therein. The dielectric rods are then placed over the helix into the respective slotted holes in the end of the shell mandrel. The alignment bushing, with corresponding holes for the rods, helix mandrel and end wire is placed over the other end of the elements and clamped into position. The clamp has an elongated hole which permits some play for the rods during the assembly and heating process.

The entire assembly is then inserted into a glass vacuum jar 74 with an inner portion of cover 12 resting on a rubber seal or gasket 76. The rounded bottom of the jar aids in preventing implosions. A vacuum pump is then operated to evacuate the chamber sufficiently to prevent excitation of residual gas and heating of the jar and shell holder. Radio frequency energy is turned on, with a current, as utilized in one instance, of 30 amps supplied to induction heating coils 7 8 for ten minutes to heat the assembly. The coil may be of 0.25 inch tubing, spaced about the dimension from the glass. The current is then increased 10 amps every five minutes up to 105 amps. The particular energy required depends upon the size of the system and frequency of operation. This heats the stainless steel shell, shell holder and shell mandrel to a temperature of between 800 to 1200 C. or preferably about 1000 C. and causes the shell diameter to expand uniformly by approximately 0.002 inch. Since stainless steel has a higher coefficient of expansion than molybdenum, of which the helix and helix mandrel are made, and alumina or beryllia ceramic, of which the rods are made, the difference in expansion is sufiicient to permit the helix and rod assembly, which would normally interfere with the shell, to be pulled into the shell. The rod and shell edges are preferably chamfered to facilitate entry. The expansion is also of a precise amount calculated to hold the helix securely inside the shell after cooling, without distorting the helix assembly and with no noticeable distortion of the circular symmetry or twisting or bowing of the shell.

The pulling is preferably accomplished by hand utilizing a special clamp, cable, and a pulley and weight system. The nut 70 is loosened slightly at the O-ring 66 to permit the pull rod of the helix insertion fixture to slip through without loss of vacuum. The lengths of the cylinder 10 and the first portion of shell holder aid in preventing twisting of the shell, provide thermal insulation for the cover of the vacuum jar and prevent the heated portion of the rod 44 from being pulled through O-rings 66. The pulling is applied with a steady even motion until the end of plug 36 of the shell holder is home against the alignment bushing. The current is then turned off gradually in about 15 minutes with the assembly being permitted to cool completely in 35 minutes, after which the device, attached to the cover, is removed from the jar. The bushing is then removed and the assembly of helix, rods and shell are slipped off the tool. The vacuum firing prevents oxidation, outgases the various elements and insures their cleanliness. Other suitable inert or reducing gas atmospheres such as hydrogen may similarly be employed. Such gases would have to be extremely dry to prevent oxidation of the stainless steel parts and burning of the graphite coatings on the ceramic rods. Other heating means such as passing current through the shell or holder, or tungsten or molybdenum coils may then be utilized. In another embodiment, the shell can be held at the end into which the rods enter and are pulled.

It may thus be seen that the present invention provides a novel, simple process and apparatus which minimizes the distortion of the shell and provides a relatively clean helix-shell assembly. In addition the positions of the elements are established precisely and securely. While only a single embodiment has been illustrated, the invention is not to be considered as limited to the exact form or use shown and many other variations may be made in the particular design and configuration without departing from the scope of the invention as set forth in the appended claims.

What is claimed is:

1. A method for assembling the shell, helix and dielectric rod members of a traveling wave tube, including mounting the shell in a support structure,

supporting the helix in axial alignment with the shell,

mounting the dielectric rods on the helix in a predetermined fixed radial relationship therewith, the outer diameter of the rods and helix minutely exceeding that of the shell,

heating the shell to expand uniformly around its circumference and along its length sufficiently to accommodate the rods and helix,

pulling the rods and helix into the shell,

cooling the shell to contract and secure the rods and helix in a fixed position with no noticeable distortion of the members, and

removing the assembled members from the support structure.

2. The method of claim 1 including,

mounting said support structure in a vacuum tight enclosure, and

positioning said shell within said support structure and the helix and rods on a shaft therein, said encolsure having a hole permitting pulling of said shaft therethrough while maintaining said vacuum.

3. A method for assembling the shell, helix and dielectric rod members of a traveling wave tube, including providing a tubular support structure and cover, positioning an insertion tool within said support, positioning the shell within said support around a first longitudinal portion of said insertion tool, the shell being held in fixed position in said support, mounting the helix on a second longitudinal portion of said tool in axial alignment with the shell, mounting and securing the dielectric rods on the helix in a predetermined radial relationship therewith, the outer diameter of the rods and helix minutely exceeding that of the shell,

mounting the support structure and insertion tool in a vacuum tight induction heating chamber,

heating said chamber to expand the shell sufiiciently to accommodate the rods and helix, the shell being formed of a material having greater expansion under heat than the helix,

pulling the tool through a vacuum tight hole in said cover to draw the rods and helix into said fixed shell, cooling the chamber to permit the shell to contract and secure the rods and helix in a fixed position therein, removing the support structure and assembled members from the chamber, and

separating the assembled members from the support structure.

4. The method of claim 3 including cleaning the members and support structure before mounting in said vacuum chamber.

5. Apparatus for assembling the shell, helix and dielectric rod members of a traveling wave tube comprising,

a vacuum chamber,

a cover for said chamber having a hole therein,

a tubular support structure secured to said cover within said chamber,

an insertion tool positioned within said support, and

having a shaft projecting through said hole, said insertlon tool having a first mandrel portion adapted to be positioned within said shell member and a second mandrel portion axially aligned with said first portion and adapted to support a helix member,

means for holding said shell in a fixed position Within said support,

means for securing said helix in a fixed position on said insertion tool and securing said rods around said helix, the diameter of said helix and rods minutely exceeding the diameter of said shell With said rods protruding radially from said securing means,

radio frequency coil means surrounding said chamber for heating said shell to expand sufficiently to accommodate the rods and helix, the shell being of a material having greater expansion under heat than the helix,

means for permitting pulling of said shaft through said hole While maintaining said vacuum, said shell contracting about said rods and helix upon cooling to maintain said rods and helix in a fixed position within said shell.

6. Apparatus for assembling the shell, helix and dielectric rod members of a traveling wave tube comprising;

a vacuum chamber,

a cover for said chamber having a hole therein,

means for supporting and holding said shell Within said chamber in a fixed position,

means for supporting and securing said helix and said rods around said helix in axial alignment with said shell, the diameter of said helix and rods being slightly larger than the diameter of said shell,

means for heating said shell to expand sufliciently to accommodate said helix and rods,

and means projecting through said hole for pulling said helix and rods into said shell while maintaining said vacuum, said shell contracting about said rods and helix upon cooling to maintain said rods and helix in a fixed position within said shell.

7. The apparatus of claim 5 wherein said tubular support includes means for preventing twisting and bowing of said shell.

8. The apparatus of claim 7 wherein said means for securing said helix and rods includes an alignment bushing and clamp having holes in one end for securing the end of said rods, helix and second mandrel, and said first mandrel portion includes an end having holes for securing the other ends of said rods and helix, said rods protruding slightly beyond the diameter of said first mandrel and bushing through slots in said holes.

References Cited UNITED STATES PATENTS 2,733,380 1/1956 Phillips et a1. 29--447 2,943,228 6/1960 =Kleinman 3153.5 2,943,382 7/1960 Harjes 29-25.l5 X 3,242,375 3/1966 Anderson et al 315--3.S

JOHN F. CAMPBELL, Primary Examiner.

D. C. REILEY, Assistant Examiner. 

